Mechanical and chemical-mechanical planarization (“CMP”) processes remove material from the surface of microfeature workpieces in the production of microelectronic devices and other products. FIG. 1 schematically illustrates a rotary CMP machine 10 with a platen 20, a carrier head 30, and a planarizing pad 40. The CMP machine 10 may also include an under-pad 50 between an upper surface 22 of the platen 20 and a lower surface of the planarizing pad 40. The under-pad 50 provides a thermal and mechanical interface between the planarizing pad 40 and the platen 20. A drive assembly 26 rotates the platen 20 (indicated by arrow F) and/or reciprocates the platen 20 back and forth (indicated by arrow G). Since the planarizing pad 40 is attached to the under-pad 50, the planarizing pad 40 moves with the platen 20 during planarization.
The carrier head 30 has a lower surface 32 to which a microfeature workpiece 12 may be attached, or the workpiece 12 may be attached to a resilient pad 34 under the lower surface 32. The carrier head 30 may be a weighted, free-floating wafer carrier, or an actuator assembly 31 may be attached to the carrier head 30 to impart rotational motion to the microfeature workpiece 12 (indicated by arrow J) and/or reciprocate the workpiece 12 back and forth (indicated by arrow I).
The planarizing pad 40 and a planarizing solution 44 define a planarizing medium that mechanically and/or chemically-mechanically removes material from the surface of the microfeature workpiece 12. The planarizing solution 44 may be a conventional CMP slurry with abrasive particles and chemicals that etch and/or oxidize the surface of the microfeature workpiece 12, or the planarizing solution 44 may be a “clean” nonabrasive planarizing solution without abrasive particles. In most CMP applications, abrasive slurries with abrasive particles are used on nonabrasive polishing pads, and clean nonabrasive solutions without abrasive particles are used on fixed-abrasive polishing pads.
To planarize the microfeature workpiece 12 with the CMP machine 10, the carrier head 30 presses the workpiece 12 facedown against the planarizing pad 40. More specifically, the carrier head 30 generally presses the microfeature workpiece 12 against the planarizing solution 44 on a planarizing surface 42 of the planarizing pad 40, and the platen 20 and/or the carrier head 30 moves to rub the workpiece 12 against the planarizing surface 42. As the microfeature workpiece 12 rubs against the planarizing surface 42, the planarizing medium removes material from the face of the workpiece 12. The force generated by friction between the microfeature workpiece 12 and the planarizing pad 40 will, at any given instant, be exerted across the surface of the workpiece 12 primarily in the direction of the relative movement between the workpiece 12 and the planarizing pad 40. A retaining ring 33 can be used to counter this force and hold the microfeature workpiece 12 in position. The frictional force drives the microfeature workpiece 12 against the retaining ring 33, which exerts a counterbalancing force to maintain the workpiece 12 in position.
The CMP process must consistently and accurately produce a uniformly planar surface on workpieces to enable precise fabrication of circuits and photo-patterns. A nonuniform surface can result, for example, when material from one area of a workpiece is removed more quickly than material from another area during CMP processing. In certain applications, the downward pressure of the retaining ring causes the under-pad and the planarizing pad to deform, creating a standing wave inside the retaining ring. Consequently, the planarizing pad removes material more quickly from the region of the workpiece adjacent to the standing wave than from the regions of the workpiece radially outward and inward from the wave. Thus, the CMP process may not produce a planar surface on the workpiece.
One approach to improve the planarity of a workpiece surface is to use a carrier head with interior and exterior bladders that modulate the downward forces on selected areas of the workpiece. These bladders can exert pressure on selected areas of the back side of the workpiece to increase the rate at which material is removed from corresponding areas on the front side. These carrier heads, however, have several drawbacks. For example, the typical bladder has a curved edge that makes it difficult to exert a uniform downward force at the perimeter. Moreover, conventional bladders cover a fairly broad area of the workpiece which limits the ability to localize the downward force on the workpiece. Furthermore, conventional bladders are often filled with compressible air that inhibits precise control of the downward force. In addition, carrier heads with multiple bladders form a complex system that is subject to significant downtime for repair and/or maintenance causing a concomitant reduction in throughput.
Another approach to improve the planarity of a workpiece surface is to use a hard under-pad to reduce the deformation caused by the retaining ring. Hard under-pads, however, increase the frequency of scratches and other defects on the workpiece because particles in the planarizing solution become trapped between the workpiece and the planarizing pad. Thus, there is a need to improve the polishing process to form uniformly planar surfaces on workpieces.